Crosslinked kraft pulp compositions and method

ABSTRACT

Pulps in accordance with certain embodiments include crosslinked cellulose fibers and have high brightness, reactivity, and intrinsic viscosity, and therefore can be well suited for use as a precursor in the production of low-color, high-viscosity cellulose derivatives. A method in accordance with the present technology includes forming a pulp from a cellulosic feedstock, bleaching the pulp, crosslinking cellulose fibers within the pulp while the pulp has a high consistency, and drying the pulp. Crosslinking the cellulose fibers can include exposing the fibers to a glycidyl ether crosslinker having two or more glycidyl groups and a molecular weight per epoxide within a range from 140 to 175. Another method in accordance with the present technology includes providing an aqueous suspension of pulp that includes chemical wood pulp fibers that were previously bleached and dried, crosslinking the fibers with such a glycidal ether crosslinker, and drying the pulp.

CROSS-REFERENCE(S) TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/112,559, filed Aug. 24, 2018, which is a continuation by-pass of PCTApplication No. PCT/US2017/019457, filed Feb. 24, 2017, which claims thebenefit of U.S. Application No. 62/299,894, filed on Feb. 25, 2016. Eachapplication is expressly incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present technology is related to cellulose products (e.g., pulp) andcellulose derivatives (e.g., cellulose ethers).

BACKGROUND

Cellulose ethers (e.g., carboxymethyl cellulose, methyl cellulose, etc.)form aqueous solutions and are available in various grades depending, inlarge part, on the viscosity of these solutions. High-grade celluloseethers that form more viscous aqueous solutions tend to be more valuablethan lower grade cellulose ethers that form less viscous aqueoussolutions. The capacity of a given cellulose ether to form a moreviscous aqueous solution is closely related to the degree ofpolymerization and/or other properties of the cellulose precursor fromwhich the given cellulose ether is produced. High-grade cellulose etheris conventionally produced from dissolving grade pulp (e.g., cottonlinters pulp), while medium grade and low grade cellulose ether isconventionally produced from lower cost wood pulps. Pulp gradesreferenced in this disclosure are further discussed in Herbert Sixta,Handbook of Pulp, Wiley-Vch (2006), which is incorporated herein byreference in its entirety. The degree of polymerization of most woodpulps does not exceed about 1,500. In contrast, dissolving grade pulpoften has a degree of polymerization of 2,400 or greater. Unfortunately,dissolving grade pulp tends to be expensive. Prior art attempts tomodify low-cost pulps for production of high-grade cellulose derivativeshave had only limited success. Accordingly, there is a need for furtherinnovation in this field.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of theclaimed subject matter will become more readily appreciated as the samebecome better understood by reference to the following detaileddescription, when taken in conjunction with the accompanying drawings,wherein:

Many aspects of the present technology can be better understood withreference to FIGS. 1 and 2 .

FIG. 1 is a flow chart illustrating a method for making pulp inaccordance with an embodiment of the present technology.

FIG. 2 is a flow chart illustrating a method for making pulp inaccordance with another embodiment of the present technology.

DETAILED DESCRIPTION

Methods for making pulp and related systems and compositions inaccordance with embodiments of the present technology can at leastpartially address one or more problems associated with conventionaltechnologies, whether or not such problems are stated herein. Forexample, methods in accordance with at least some embodiments of thepresent technology allow low-cost pulp to serve as a precursor in theproduction of high-grade cellulose ethers and/or other cellulosederivatives (e.g., cellulose esters). Kraft pulp, for example, is farless expensive and more widely available than dissolving grade pulp.When standard Kraft pulp is used as a precursor for production ofcellulose ethers, however, the resulting cellulose ethers tend to be lowgrade.

Desirable properties in a pulp for use in producing cellulosederivatives include high brightness and high viscosity. High brightnessin a pulp can cause cellulose derivatives produced from the pulp to havelittle or no color. In many products made with cellulose derivatives(e.g., medical fabrics, foods, drywall, etc.), it is undesirable for thecellulose derivatives to impart color to the products. Similarly, inthese and other applications, it tends to be useful for cellulosederivatives to have high viscosity while still remaining soluble inwater. Accordingly, cellulose derivatives with little or no color andhigh viscosity tend to be more expensive than cellulose derivatives withsignificant color and low viscosity. High reactivity in a pulp increasesthe efficiency of functionalizing reactions (e.g., etherification) usedto convert the pulp into cellulose derivatives. The reactivity of a pulpis closely related to the degree to which the surfaces of cellulosefibers in the pulp contact water molecules in which a functionalizingagent is distributed to the surfaces. Greater contact between thesesurfaces and water molecules also increases the degree to which pulpholds water. Accordingly, pulps with higher water retention values tendto also be pulps with higher reactivity.

Several conventional processes have, to some degree, been successful inincreasing the capacity of Kraft pulp to produce cellulose ethers thatform high viscosity aqueous solutions. Unfortunately, these conventionalprocesses have invariably done so at the expense of other desirableproperties in the cellulose ethers and/or at the expense of processyields. For example, some conventional processes include reducing oreliminating bleaching in the Kraft process. The pulps resulting fromthese processes tend to have high lignin contents and, correspondingly,low brightness. This leads to undesirable color in cellulose ethersproduced from these pulps. As another example, some conventionalprocesses include increasing removal of hemicellulose from Kraft pulp.These processes, however, have low yields due to removal of the bulkthat hemicellulose provides. Moreover, the pulps resulting from theseprocesses tend to have low reactivity due to conversion of constituentcellulose from cellulose-I to cellulose-II. Conventional crosslinkingreactions also typically reduce the water retention value (andreactivity) of a pulp. Even allowing for low brightness, low yield,and/or low reactivity, conventional processes for modifying low-costpulp have still not been able to produce pulp suitable for producingcellulose ethers that form aqueous solutions with viscosities as high asthose of cellulose derivatives produced from high viscosity dissolvinggrade ether pulps.

Methods in accordance with at least some embodiments of the presenttechnology include crosslinking pulp at relatively high consistency(e.g., consistency greater than or equal to 12%). These methods canfurther include crosslinking pulp with a crosslinker selected toincrease the water retention value of the pulp. Crosslinkers well suitedfor this purpose include, for example, glycidyl ether crosslinker havingtwo or more glycidyl groups and a molecular weight per epoxide within arange from 140 to 175. Crosslinking pulp at relatively high consistencyand using this type of crosslinker has surprisingly been found to causelow-cost pulp (e.g., Kraft pulp) to become suitable for production ofhigh-grade cellulose derivatives with little or no associated reductionin brightness, yield, reactivity, and/or other desirable pulpproperties.

Conventional, non-crosslinked Kraft pulps tend to have lower reactivitythan other chemical pulps, such as sulfite pulps (i.e., pulps made byextracting lignin from wood primarily using salts of sulfurous acid). Inat least some embodiments of the present technology, however,crosslinked Kraft pulp has relatively high reactivity. Solely by way oftheory and without wishing to be limited to such theory, this may be dueto the presence of crosslinks that add extra space uniformly betweencellulose chains. Longer chain crosslinkers (e.g., polyglycidyl ether)may produce crosslinked pulp with higher reactivity than shorter chaincrosslinkers (e.g., 1,3-dichloro-2-hydroxypropanol (DCP)) under similarcrosslinking conditions. Pulps crosslinked with longer chaincrosslinkers may have crystallinity indexes lower than those of startingpulps and much lower than those of dissolving grade sulfite wood pulpand cotton linters pulp. Crosslinking Kraft pulp instead of sulfite pulpfor high viscosity ether applications can be advantageous in at leastsome cases because Kraft is the dominant pulping process, has higheryield (higher hemicellulose content), lower cost and is moreenvironmentally friendly than sulfite pulping processes.

In conventional Kraft processing, pulp is maintained at relatively lowconsistency (e.g., consistency less than or equal to 10%). As theconsistency of pulp increases, it becomes more difficult to flow thepulp through pipes and to mix the pulp. Therefore, any crosslinking inconventional processes for modifying Kraft pulp to increase itspotential for production of high-grade cellulose derivatives has alsobeen carried out at relatively low consistency. One of the surprisingdiscoveries associated with at least some embodiments of the presenttechnology is that increasing the consistency of pulp duringcrosslinking can increase the water retention value of the crosslinkedpulp. With this and/or other discoveries associated with at least someembodiments of the present technology, it is finally possible to produceKraft pulp that is a true substitute for and/or a suitable extender ofexpensive dissolving grade pulp in production of high-grade cellulosederivatives.

Specific details of methods for making pulp and related systems andcompositions in accordance with several embodiments of the presenttechnology are described herein with reference to FIGS. 1 and 2 .Although the methods and related systems and compositions may bedisclosed herein primarily or entirely in the context of modifying Kraftpulp for production of cellulose derivatives, other contexts in additionto those disclosed herein are within the scope of the presenttechnology. For example, suitable features of described methods,systems, and compositions can be implemented in the context of sulfitepulp or even in the context of dissolving grade pulp. As anotherexample, suitable features of described methods, systems, andcompositions can be implemented in the context of modifying Kraft orother pulp for uses other than production of cellulose derivatives, suchas production of specialty paper products.

It should be understood, in general, that other methods, systems, andcompositions in addition to those disclosed herein are within the scopeof the present technology. For example, methods, systems, andcompositions in accordance with embodiments of the present technologycan have different and/or additional operations, components,configurations, etc. than those disclosed herein. Moreover, a person ofordinary skill in the art will understand that methods, systems, andcompositions in accordance with embodiments of the present technologycan be without certain operations, components, configurations, etc.disclosed herein without deviating from the present technology.

Test Methods and Acronyms

-   -   AOX: adsorbable organic halides, as determined by EPA Method        1650A.    -   Ash Content: Determined by TAPPI T 211 om-07.    -   ASTM: American Society for Testing and Materials    -   Brightness: Determined by TAPPI T 525 om-12.    -   Capillary Viscosity: Determined by TAPPI T 230 om-99.    -   Carboxyl Content: Determined by TAPPI T 237 om-08    -   Centrifugal Capacity: Determined by the corresponding method        disclosed in U.S. Pat. No. 8,039,683, which is incorporated        herein by reference in its entirety.    -   CMC: carboxymethyl cellulose.    -   Coarseness: Determined by the corresponding method disclosed in        U.S. Pat. No. 6,685,856, which is incorporated herein by        reference in its entirety.    -   Crystallinity Index: Determined by the corresponding method of        Lionetto et al., “Monitoring Wood Degradation during Weathering        by Cellulose Crystallinity,” Materials, 5, 1910-1922 (2012),        which is incorporated herein by reference in its entirety.    -   Cuen Solubility: Solubility in cupriethylenediamine under the        conditions of TAPPI T 254 cm-00.    -   Curl Index: Determined by the corresponding method disclosed in        U.S. Pat. No. 6,685,856, which is incorporated herein by        reference in its entirety.    -   Degree of Polymerization: The number of D-glucose monomers in a        cellulose molecule as measured by ASTM-D1795-96. Average degree        of polymerization refers to the average number of D-glucose        molecules per cellulose polymer in a population of cellulose        polymers.    -   Degree of Substitution (DS): Determined by ASTM D 1439-03.    -   DWP: dissolving wood pulp.    -   Falling-Ball (FB) Viscosity: Determined by TAPPI T 254 cm-00.    -   Freeness: Canadian Standard Freeness as determined by TAPPI T        227 om-04.    -   Free Swell: Determined by the corresponding method disclosed in        U.S. Pat. No. 8,039,683, which is incorporated herein by        reference in its entirety.    -   Hemicellulose Content: The sum of mannan and xylan content as        determined by the method described in Examples 6 and 7 of U.S.        Pat. No. 7,541,396, which is incorporated herein by reference in        its entirety. This test is based on TAPPI T 249 cm00 with        analysis by Dionex ion chromatography.    -   HPLC: High Performance Liquid Chromatography.    -   Intrinsic Viscosity (IV): Determined by ASTM D1795-96.    -   ISO: International Organization for Standardization.    -   Kappa Number: Determined by ISO 302:2004.    -   Kink Angle: Determined by the corresponding method disclosed in        U.S. Pat. No. 6,685,856, which is incorporated herein by        reference in its entirety.    -   Kink Index: Determined by the corresponding method disclosed in        U.S. Pat. No. 6,685,856, which is incorporated herein by        reference in its entirety.    -   Lignin Content: Determined by the method described in Examples 6        and 7 of U.S. Pat. No. 7,541,396, which is incorporated herein        by reference in its entirety.    -   MCA: monochloroacetic acid.    -   NCASI: National Council for Air and Stream Improvement.    -   Oven Dried (OD): Dried to less than or equal to 7% moisture by        weight.    -   R18: Measured by TAPPI T 235 cm-00.    -   Resultant CMC Viscosity: Refers to the viscosity of a 0.5%        solution of resultant CMC according to the Resultant CMC Testing        Method below.    -   Resultant CMC Color: See Resultant CMC Testing Method below.    -   Resultant CMC Turbidity: See Resultant CMC Testing Method below.    -   TAPPI: Technical Association of the Pulp and Paper Industry.    -   Transition Metal Content: Determined by EPA SW-856 method 3050,        200.8.    -   US EPA: United States Environmental Protection Agency.    -   Water Retention Value (WRV): Determined by TAPPI T UM256M        (2011).    -   Wet Bulk: Determined by the corresponding method disclosed in        U.S. Pat. No. 8,722,797, which is incorporated herein by        reference in its entirety.    -   WPE: weight per epoxide.

Resultant CMC Testing Method

Throughout this disclosure, the properties of pulp may be characterizedin terms of “resultant CMC” properties. These are properties of CMC thatthe pulp can be used to produce, with CMC serving as a representativeexample of a cellulose derivative. It should be understood that CMC isnot the only cellulose derivative that pulps in accordance withembodiments of the present technology may be used to produce. ResultantCMC properties of a given pulp described herein are determined by thefollowing procedure. Additional details regarding this procedure can befound in Nevell T. P. and Zeronian S., Cellulose Chemistry and itsApplications, Chapter 15-Cellulose Ethers (1985), which is incorporatedherein by reference in its entirety.

First, determine the degree of substitution of the pulp. If the degreeof substitution of the pulp is at least 1.0, proceed as specified below.If the degree of substitution of the pulp is less than or equal to 1.0,proceed as specified below, but use 6.4 mL (instead of 8.0 mL) of 30%NaOH solution and 2.9 g (instead of 3.6 g) of MCA. Slurry a 3 g (ovendried) sample of the pulp (fiberized) in 80 mL isopropanol. Add 8.0 mLof 30% NaOH solution over a 3 minute period. Stir the suspension for 1hour at 20° C. Add 3.6 g of MCA (as 15.2 mL of a solution of 23.6 gMCA/100 mL isopropanol) over a 3 minute period. Raise the temperature to55° C. in 25 minutes and continue stirring for 3.5 hours. Drain theresulting fibrous CMC and wash with 70% ethanol. Bring the sample toneutrality (pH 7.0) with acetic acid and then filter. Wash the filtercake again with 70% ethanol at 20° C. and filter. Repeat washing andfiltering for one more wash with 70% ethanol at 20° C. and then for 3more washes with 100% denatured ethanol at 20° C. Air dry the sample to70-85% solids to form the resultant CMC. Test a 0.5% solution of theresultant CMC with a Brookfield viscometer using spindle 2 and 50 RPM at20° C. according to ASTM method D2196-99 to determine resultant CMCviscosity. Test a 0.5% solution of the resultant CMC by US EPA method180.1 to determine resultant CMC turbidity and a 0.013% solution of theresultant CMC by NCASI Method TB253 to determine resultant CMC color.

Starting Materials

Examples of suitable starting materials for making pulp in accordancewith embodiments of the present technology include wood and recycledpaper. In at least some embodiments, the starting material is neverdried. In at least some embodiments, the starting material is dried. Inthe wood pulping industry, trees are conventionally classified as eitherhardwood or softwood. Pulp for use as starting material can be derivedfrom a softwood or hardwood tree species. Examples of suitable softwoodtree species include fir (e.g., Douglas fir and balsam fir), pine (e.g.,eastern white pine and loblolly pine), spruce (e.g., white spruce),larch (e.g., eastern larch), cedar, and hemlock (e.g., eastern andwestern hemlock). Examples of suitable hardwood species include acacia,alder (e.g., red alder and European black alder), aspen (e.g., quakingaspen), beech, birch, oak (e.g., white oak), gum trees (e.g., eucalyptusand sweetgum), poplar (e.g., balsam poplar, eastern cottonwood, blackcottonwood, and yellow poplar), gmelina and maple (e.g., sugar maple,red maple, silver maple, and bigleaf maple).

Wood from softwood or hardwood species generally includes three majorcomponents: cellulose, hemicellulose, and lignin. Cellulose makes upabout 50% of the woody structure of plants and is an unbranched polymerof D-glucose monomers. Individual cellulose polymer chains associate toform thicker microfibrils that, in turn, associate to form fibrilsarranged into bundles. The bundles form fibers that are visible ascomponents of the plant cell wall when viewed at high magnificationunder a light microscope or a scanning electron microscope. Cellulose ishighly crystalline as a result of extensive intramolecular andintermolecular hydrogen bonding. Hemicellulose is a heterogeneous groupof low molecular weight carbohydrate polymers such as xylan and mannanthat are associated with cellulose in wood. Hemicelluloses areamorphous, branched polymers, in contrast to cellulose which is a linearpolymer. Lignin is a complex aromatic polymer and comprises about 20% to40% of wood where it occurs as an amorphous polymer.

Use of Never-Dried Pulp and a Modified Kraft Process

As noted above, some embodiments involve the use of never-dried pulp asa starting material for the production of pulp in accordance with thepresent disclosure. In most pulp mills, the pulp produced by thechemical pulping process (such as the Kraft process) is followed by ableaching process to further delignify and whiten the pulp. The bleachedpulp is then dewatered and dried for shipping. Embodiments in whichnever-dried pulp is used may involve a modified Kraft process, in whichthe pulp produced by Kraft pulping is then treated with a crosslinkerprior to drying, to produce a crosslinked pulp. For example, the pulpmay be treated with a crosslinker during the bleaching process, such asbetween two bleaching stages, or during or after a selected bleachingstage. This approach may be advantageous in that the crosslinking stepmay be selectively integrated into the standard Kraft process employedat a mill, allowing swing production capacity. Moreover, crosslinkerssuitable for the production of pulps in accordance with the presentdisclosure generally require an aqueous medium, which is provided in thebleaching process.

An illustrative example of a modified Kraft process is described belowin connection with FIG. 1 .

In general, Kraft processing involves chemically digesting cellulosicfeedstock (e.g., wood chips) at elevated temperature and pressure inwhite liquor, which is an aqueous solution of cooking chemicals (e.g.,sodium sulfide and sodium hydroxide). The cooking chemicals dissolvelignin that binds together cellulose fibers within the feedstock. Whenthis chemical digestion is complete, the pulp is transferred to anatmospheric tank known as a “blow tank.” The contents of the blow tankare then sent to pulp washers, where the spent cooking chemicals areseparated from the pulp. The pulp then proceeds through various stagesof washing and bleaching, after which it is pressed and dried into afinished product.

The Kraft process is designed to recover the cooking chemicals and heat.For example, spent cooking chemicals and pulp wash water can be combinedto form a weak black liquor that is concentrated in a multiple-effectevaporator system to about 55% solids. The black liquor can then befurther concentrated to 65% solids in a direct-contact evaporator bybringing the liquor into contact with flue gases from a recovery furnaceor in an indirect-contact concentrator. The strong black liquor can thenbe fired in a recovery furnace. Combustion of organics dissolved in theblack liquor can provide heat for generating process steam and forconverting sodium sulfate to sodium sulfide. Inorganic chemicals presentin the black liquor may collect as a molten smelt at the bottom of thefurnace. The smelt can be dissolved in water to form green liquor, whichcan then be transferred to a causticizing tank where quicklime (calciumoxide) can be added to convert the solution back to white liquor forreturn to the digester system. A lime mud precipitate from thecausticizing tank can be calcined in a lime kiln to regeneratequicklime.

FIG. 1 is a flow chart illustrating an example method 100 for makingpulp in accordance with an embodiment of the present technology. In theillustrated embodiment, the method 100 is based on a Kraft process. Inother embodiments, counterparts of the method 100 can be based on othersuitable processes. With reference to FIG. 1 , the method 100 caninclude a pulping process 102 and a post-pulping process 104. Within thepulping process 102, the method 100 can include loading chips (block106) and pre-steaming the chips (block 108). Steam at atmosphericpressure can be used to preheat the chips and drive off air to enhanceliquor penetration. After pre-steaming, the method 100 can includeadding chemicals (e.g., NaOH, Na2S, and/or other suitable chemicals) tothe chips (block 110). For example, the chemicals can be added as acooking liquor. The wood chips and cooking liquor can then be fed into adigester. Within the digester, the cooking liquor can be allowed toimpregnate the wood chips (block 112). Good penetration of the cookingliquor can promote uniform cooking of the wood chips.

After impregnation, the method 100 can include cooking the wood chipsand cooking liquor in co-current (block 114) and counter-current (block116) liquid contact. In either operation, the cooking liquor and chipscan be brought to temperature. Next, wash liquor can be introduced intothe bottom of the digester such that it flows counter-currently to thecooked pulp (block 118). Cooking can end when the pulp encounters thecooler wash liquor. After digester washing, the digester contents can beblown (block 120). Digester blowing can involve releasing wood andliquor at atmospheric pressure. The release can occur with a sufficientamount of force to cause fiber separation. If desired, the blow tank canbe equipped with heat recovery equipment to reduce operating expenses.Finally, the pulp can be sent from the blow tank to an external pulpwasher for separation of black liquor from the pulp (block 122).

Following the pulping process 102, the pulp can be bleached andcellulose fibers within the pulp can be crosslinked. In a standard Kraftprocess, bleaching occurs without crosslinking. Bleaching typically doesnot cause a substantial reduction of the hemicellulose content of apulp. Instead, bleaching involves further removal of residual ligninwith an attendant decrease in pulp fiber length and viscosity. Duringbleaching, pulp can be treated with various chemicals at differentstages in a bleach plant. The stages can be carried out in vessels ortowers of conventional design. Bleaching typically occurs as a sequenceof operations, such as one or more bleaching stages with differentbleaching agents (e.g., oxygen, chlorine dioxide, etc.), extractionstages, other treatment stages, and so forth. The bleaching sequence maybe identified in terms of the order of the operations performed in thesequence. For example, one example of a bleaching sequence is O-D-E-D.Such a bleaching sequence includes an oxygen bleaching stage (an “Ostage”), followed by a first chlorine dioxide bleaching stage (a “Dstage”), followed by an extraction stage (an “E stage,” or “EOP stage”in which bleaching chemicals such as peroxide (“P”) and/or oxygen (“O”)are mixed with caustic for removing lignin), and a second D stage.Several additional examples of bleaching processes are described in U.S.Pat. Nos. 6,331,354, and 6,605,350, which are incorporated herein byreference in their entireties.

The post-pulping process 104 can include first bleaching the pulp withoxygen (block 124). Bleaching pulp with oxygen tends to be less specificfor the removal of lignin than bleaching pulp with chlorine dioxide. Theoxygen bleaching can take place in an oxygen reactor under pressure.Suitable oxygen reactors and associated oxygen bleaching processes aredescribed in U.S. Pat. Nos. 4,295,925, 4,295,926, 4,298,426, and4,295,927, which are incorporated herein by reference in theirentireties. The amount of oxygen added to the pulp can be within a rangefrom 50 to 80 pounds per ton of pulp. The temperature during the oxygenbleaching can be within a range from 100° C. to 140° C.

After oxygen bleaching the pulp, the method 100 can include crosslinkingcellulose fibers within the pulp (block 126). In at least some cases,this includes adding a crosslinker to the pulp and allowing acrosslinking reaction to occur before further processing the pulp. Thecrosslinker can be selected to form relatively strong crosslinks (e.g.,ether crosslinks instead of ester or ionic crosslinks). Relativelystrong crosslinks can be preferable to weaker crosslinks, for example,so that the crosslinks will be less likely to be disrupted byfunctionalizing reactions (e.g., etherification) used to form cellulosederivatives. The crosslinker can be added at a weight ratio relative tothe pulp of greater than or equal to 2:100, greater than or equal to3:100, greater than or equal to 5:100, or greater than or equal toanother suitable lower threshold. The upper threshold can be a maximumamount of crosslinker that can be used without causing resultant CMCfrom the pulp to become insoluble in water. In at least some cases, acatalyst (e.g., NaOH, zinc tetrafluoroborate, Zn(BF4)2) is presentduring crosslinking. In addition or alternatively, a surfactant can bepresent during crosslinking, such as to promote crosslinker dispersionand penetration. A surfactant can be especially useful in conjunctionwith a hydrophobic crosslinker.

Suitable crosslinkers include ethers, such as glycidyl ethers having twoor more glycidyl groups. For example, the crosslinker can include afirst glycidyl group, a second glycidyl group, and either three or fourlinear chain carbon atoms between the first and second glycidyl groups.In addition or alternatively, the crosslinker can have weight averagemolecular weight less than or equal to 500 (e.g., within a range from174 to 500). Furthermore, when the crosslinker is an epoxide, thecrosslinker can have a weight per epoxide less than or equal to 175(e.g., within a range from 140 to 175). The crosslinker can have aviscosity of less than or equal to 500 cP at 25° C. In at least someembodiments, the crosslinker is at least partially insoluble in water.This property can be useful, for example, to increase contact betweenthe crosslinker and cellulose fibers during the crosslinking reaction.Specific examples of suitable crosslinkers include trimethylolethanetriglycidyl ether, 1,4-butanediol diglycidyl ether, glycerol diglycidalether, neopentyl glycol diglycidyl ether, glycerol polyglycidyl ether,glycerol triglycidyl ether, ethyleneglycol diglycidyl ether, andtrimethylol propane triglycidyl ether, among others.

During crosslinking, the pulp can have a temperature within a range from50° C. to 85° C. Furthermore, the pulp can have a pH within a range from9 to 14. As discussed above, crosslinking while the consistency of thepulp is relatively high can be useful to increase the ability of thepulp to produce high-grade cellulose derivatives. The consistency of thepulp during all or a portion (e.g., at least 50% by time) of thecrosslinking can be at least 12% (e.g., within a range from 12% to 30%)or at least 15% (e.g., within a range from 15% to 30%). For example, theconsistency of the pulp can be increased (e.g., by pressing off water)before crosslinking. When further pulp processing is to occur aftercrosslinking, the consistency of the pulp can be decreased (e.g., byadding water) after crosslinking. Due to the relatively high consistencyand/or other factors, the crosslinking can increase the reactivity (asmeasured by water retention value) and alkaline resistance (as measuredby R18) of the pulp. In contrast, conventional crosslinking processes atleast typically reduce or do not affect one or both of these desirableproperties.

Crosslinking pulp in accordance with embodiments of the presenttechnology can be used in combination with other techniques forincreasing the ability of pulp to produce high-grade cellulosederivatives. For example, the cooking described above in the pulpingprocess 102 can be relatively mild. With relatively mild cooking, lesslignin may be removed from the pulp than would otherwise be the case.After mild cooking, the pulp may have a kappa number from 25-35indicating the presence of significant residual lignin. As anotherexample, the bleaching and extraction described below in thepost-pulping process 104 can be relatively mild. Unlike modifying theKraft process by adding strong caustic extraction and prehydrolysis, theaforementioned modifications to the Kraft process can incrementallyimprove the ability of Kraft pulp to produce high-grade cellulosederivatives without unduly compromising brightness, yield, and/orreactivity.

After crosslinking cellulose fibers within the pulp, the method 100 caninclude bleaching the pulp with chlorine dioxide a first time (block128). Chlorine dioxide bleaching tends to be more selective than oxygenbleaching for removing lignin. The amount of chlorine dioxide added tothe pulp can be within a range from 20 to 30 pounds per ton of pulp. Thetemperature during the first chlorine dioxide bleaching can be within arange from 50° C. to 85° C. After chlorine dioxide bleaching the pulpthe first time, the method 100 can include extraction (block 130) toremove lignin from the pulp. The extraction can include adding hydrogenperoxide or another suitable caustic to the pulp. The amount of hydrogenperoxide added to the pulp can be within a range from 20 to 100 poundsper ton of pulp. The temperature during extraction can be within a rangefrom 75° C. to 95° C. In contrast to strong caustic extraction forremoving hemicellulose, extraction for removing lignin can be relativelymild. For example, the extraction can be one that does not change thecrystal structure of the cellulose fibers.

With reference again to FIG. 1 , after extraction, the method 100 caninclude bleaching the pulp with chlorine dioxide a second time (block132). The amount of chlorine dioxide added to the pulp can be within arange from 10 to 30 pounds per ton of pulp. The temperature during thesecond chlorine dioxide bleaching can be within a range from 60° C. to90° C. The method 100 can further include additional operations otherthan the operations specifically identified in FIG. 1 . For example,after any of the operations in the post-pulping process 104, the method100 can include washing the pulp. This can be useful, for example, toremove carryover, to increase pulp consistency, and so forth.

For example, a washing operation can be used to increase the pulpconsistency after oxygen bleaching the pulp and before crosslinking thepulp. A washing operation after crosslinking the pulp can reduce orremove carryover. An example of carryover is adsorbable organic halidesor halogenated compounds, which is typically referred to as AOX, andexpressed as an AOX content of AOX level, usually in parts per million(ppm). Such organic halogens may be introduced by or with variousreactants and additives during pulp production, even if elementalchlorine free bleaching processes are used. For example, thecrosslinkers discussed above may be sources of such organic halogens.Some crosslinking reagents may contain reaction intermediates from theirrespective synthesis processes. Often, these reaction intermediates arenot effective crosslinkers, and during the crosslinking process in whichthe pulp fibers are exposed to the crosslinking reagents, theintermediates will stay with the pulp fibers and thereby introduceorganic halogens to the crosslinked pulp. Although the AOX content isgenerally not high enough to adversely affect the ability of the pulp toproduce suitable cellulose ethers, special handling requirements for AOXand other carryover materials in waste water from processing may addexpense. As such, it may be desirable to reduce the AOX level andcontent of other carryover materials in the crosslinked pulp, such as tothe level normally associated with dissolving grade market pulps.

The washing operation, or operations, are generally done with water, butany suitable washing reagents may be used. AOX content in thecrosslinked pulp may be more effectively reduced by washing with waterat higher temperatures, as compared to lower temperatures. Although notwishing to be bound by theory, this is thought to be related to the lowwater solubility of the crosslinkers discussed herein and their expectedintermediates, in that higher temperature washing enhances their removalfrom the pulp and the organic halogens associated therewith. Somemethods thus include washing the crosslinked pulp fibers with water at atemperature between 30° C. and 80° C. to remove residual crosslinker.The washing operation(s) may optionally include or be accompanied byhigh-energy dispersion (e.g., disintegration in a Waring blender orsimilar mixer) of the fibers, which is thought to promote additionalreaction between residual crosslinker and the pulp fibers, also reducingAOX content associated with the presence of the crosslinker. Crosslinkedpulps washed in this manner can exhibit a significant reduction of AOXcontent (e.g. up to 90%, up to 95%, or even up to 97%) as compared tocrosslinked pulps that are not. In some embodiments, the pulp prior towashing can exhibit an AOX content of up to 1000 ppm or higher, whereasthe AOX content of the pulp after washing may be 200 ppm or less, e.g.100 ppm or less, 50 ppm or less, 20 ppm or less, etc.

Although crosslinking in the embodiment illustrated in method 100 occursafter oxygen bleaching and before the chlorine dioxide bleaching, inother embodiments, crosslinking can occur at another point in acounterpart of the post-pulping process 104 as described below. Thebleaching and extraction operations can also be rearranged or removed inother embodiments. If “X” is defined as a crosslinking operation,post-pulping methods in accordance with several embodiments of thepresent technology can be characterized as: O-X-D-E-D (FIG. 1 ),O-D-X-E-D, O-D-E-X, O-D-E-X-D, O-D-E-D-X, D-X-E-D-E-D, D-E-X-D-E-D,D-E-D-X-E-D, D-E-D-E-X-D, D-E-D-E-D-X, D-X-E-E-D, D-E-X-E-D, D-E-E-X-D,or D-E-E-D-X, among numerous other suitable permutations. Furthermore,the crosslinking can occur during oxygen bleaching, chlorine dioxidebleaching, and/or extraction. Thus, post-pulping methods in accordancewith several more embodiments of the present technology can becharacterized as: O/X-D-E-D, O-D/X-E-D, O-D-E/X-D, O-D-E/X, O-D-E-D/X,D/X-E-D-E-D, D-E/X-D-E-D, D-E-D/X-E-D, D-E-D-E/X-D, D-E-D-E-D/X,D/X-E-E-D, D-E/X-E-D, D-E-E/X-D, D-E-E-D/X, among numerous othersuitable permutations.

After the bleaching process 104, the method 100 can include processingthe pulp for use, sale, and/or transport (block 134). For example, thepulp can be further washed, dried (e.g., flash dried), pressed,containerized, and/or otherwise processed to put the pulp into asuitable form (e.g., sheet, bale, roll, etc.) for use, sale, and/ortransport. The pulp can have a basis weight from 500 to 1200 g/m2 and/ora density of 0.2 to 0.9 g/cm3.

Use of Dried Pulp

In the generally accepted model of the cell wall structure of woodymaterials, microfibrils of cellulose, together with lignin-hemicellulosematrix surrounding them, form a lamella structure. The intralamellarspace within the lignin-hemicellulose matrix and between microfibrilsforms a fraction of small pores sometimes referred to as micropores. Inchemical pulping, most of the lignin and a certain amount ofhemicellulose is dissolved, leaving a space between lamellae which formsrelatively large pores sometimes referred to as macropores.

The fiber pore structure is altered when pulp is dried. In particular,when water is removed from pulp during the drying process, a largefraction of the pores partially or completely collapse, resulting in aloss of pore volume. Moreover, hydrogen bonds form between and withinthe cell walls, which maintains many of the collapsed pores in thatstate permanently. This phenomenon is known as hornification, and tendsto result in an irreversible reduction in the WRV of the pulp. In otherwords, never-dried pulp generally possesses a higher WRV as compared tothe pulp after drying (even if the dried pulp is then rewetted).

Although the formation of hydrogen bonds upon drying pulp tends toenhance some properties of pulp, such as certain strength andflexibility characteristics that are useful in absorbent applications,dried pulp is generally expected to be less reactive as compared tonever-dried pulp. The reduction in reactivity is due to the loss of porevolume due to hornification, which reduces and/or slows penetration ofreactants into the fiber structure.

As such, it would be expected that dried pulp would not be a suitablestarting material for producing pulps in accordance with the presentdisclosure, as the dried pulp fibers would not be expected to possesssufficient void volume for the crosslinker to penetrate and achievecrosslinking. However, it has been found that dried Kraft pulps aresuitable starting materials to produce the pulps in accordance with thepresent disclosure, that is, crosslinked pulps that can producecellulose ethers that form high viscosity aqueous solutions and thatpossess the various characteristics described herein (e.g., WRV,brightness, R18, lignin and cellulose-II content, etc.).

An approach in which dried pulp is used as a starting material may beadvantageous in that conventional Kraft market pulp is widely available,and is generally less expensive and easier to transport than never-driedpulp. Moreover, the use of dried pulp may allow the crosslinking processto be done separately from (instead of in connection with) the pulpbleaching process, such as performing the crosslinking at a differentfacility and/or at a different time from where (or when) the startingmaterial is produced.

An illustrative example of a method in which dried pulp is used as astarting material is described below in connection with FIG. 2 , a flowchart illustrating a method 200 for making pulp in accordance withanother embodiment of the present technology. As a number of theconcepts invoked in the description of method 200 are discussed indetail above with respect to method 100, the discussion with regard tomethod 200 may be understood by reference to such concepts, which areintended to have the same scope as detailed elsewhere herein, eventhough they may not be described in the same level of detail in theexplanation below. Briefly, the method 200 begins, at 202, withproviding an aqueous suspension of pulp that includes chemical wood pulpfibers that were previously bleached and dried. At 204, the methodincludes crosslinking the chemical wood pulp fibers, and at 206, themethod includes drying the pulp after crosslinking. The pulp is thenprocessed for use, sale, or transport as market pulp, at 208.

Method 200 is based on a Kraft process, in that the dried and bleachedchemical wood pulp fibers may be chemical wood pulp fibers producedaccording to a Kraft pulp process and then bleached (withoutcrosslinking), as described in connection with method 100, butcounterparts of method 200 may use any suitably dried and bleachedchemical wood pulp fibers. The aqueous suspension in 202 may be producedin any suitable manner, such as by slurrying the pulp in water at adesired consistency in a mixing device.

Once the aqueous suspension of pulp is provided, the method 200 caninclude crosslinking the chemical wood pulp fibers with a crosslinker.As in method 100, this may include adding a crosslinker (and optionallya catalyst) to the aqueous suspension and allowing a crosslinkingreaction to occur before further processing the pulp. The crosslinker isa glycidal ether crosslinker having two or more glycidal groups, asdescribed above with regard to method 100, and the consistency of theaqueous suspension during all or a portion (e.g., at least 50% by time)of the crosslinking can be at least 12% (e.g., within a range from 12%to 30%) or at least 15% (e.g., within a range from 15% to 30%). Othercharacteristics of the crosslinker and the crosslinking steps may be asdescribed above as in method 100.

After crosslinking, the method 200 includes drying the pulp, at 206. At208, the method can include processing the pulp for use, sale, and/ortransport. Various pulp processing operations, such as drying, pressing,containerizing, and so forth, are described above with respect to method100.

Also similar to method 100, method 200 may include additional operationsother than those specifically identified in FIG. 2 . For example, themethod 200 may include washing the pulp, such as after crosslinking thechemical wood pulp fibers and prior to drying the pulp. This can beuseful to remove carryover, for example, by reducing the AOX content ofthe pulp and/or removing residual crosslinker and other chemicals,increase pulp consistency, and so forth. For AOX reduction, washing maybe done with water at a temperature between 30° C. and 80° C.,optionally in connection with high-energy dispersion of the pulp.

Pulp Blends

In some embodiments, the crosslinked pulp—that is, a pulp producedaccording to aspects of the present disclosure, such as by method 100and/or 200—is combined with another pulp before being dried. Pulps inaccordance with at least some embodiments of the present technology arewell suited for use as pulp extenders that reduce the amount ofexpensive dissolving grade pulp needed for producing a given cellulosederivative product without compromising the viscosity or other desirableproperties of the product. For example, the crosslinked pulp can beblended with another pulp (e.g., a dissolving grade pulp having acellulose content greater than 90% by oven dried weight) such that thecrosslinked pulp makes up at least 20% (e.g., at least 30%) by celluloseoven-dried weight of a resulting blended pulp. In other embodiments, thecrosslinked pulp can be used without being blended with another pulp.

Crosslinked Pulp Properties

Pulp in accordance with embodiments of the present technology can haveone or more of the following properties:

AOX content less than or equal to 200 ppm (e.g., less than or equal to100 ppm, 50 ppm, and/or 20 ppm).

Basis weight greater than or equal to 500 g/m2 and/or less than or equalto 1200 g/m2.

Brightness greater than or equal to 75% (e.g., greater than or equal to80% or 85%) and/or less than or equal to 92% (e.g., less than or equalto 88.5%). For example, the brightness can be within a range from 80% to88%.

Cellulose-II structure negligible (e.g., at least substantially nocellulose-II structure) as determined by x-ray crystallography.

Crystallinity index less than or equal to 80% (e.g., less than or equalto 75%).

Cuen solubility less than complete (e.g., insoluble or only partiallysoluble).

Density greater than or equal to 0.20 g/cm3 (e.g., greater than or equalto 0.55 or 0.60 g/cm3).

Falling ball viscosity greater than or equal to 200 cP (e.g., greaterthan or equal to 200, 300, 330, 500, 800, 1,000, 1,400, 2,000, or 3,000cP). At very high degrees of crosslinking, the falling ball viscosity ofcrosslinked pulps in accordance with at least some embodiments of thepresent technology may be low, but the resultant CMC viscosity for thesepulps may still be very high. Solely by way of theory, and withoutwishing to be bound to such theory, the molecular structure of thecellulose may change from linear to highly branched at high degrees ofcrosslinking. Cellulose having a highly branched structure may have alow falling ball viscosity, but still be capable of forming high-gradeether.

Freeness greater than or equal to 700 mL.

Hemicellulose content greater than or equal to 6% (e.g., greater than orequal to 10%, 13.5%, or 15.5%) and/or less than or equal to 20% (e.g.,less than or equal to 18%, 16%, or 14%) by weight. For example, thehemicellulose content can be within a range from 6% to 20%, within arange from 7% to 17%, or within a range from 8 to 15% by weight.

Intrinsic viscosity greater than or equal to 1,150 mL/g (e.g., greaterthan or equal to 1,300, 1,400, 1,500, or 2,100 mL/g).

Lignin content less than or equal to 1.0% (e.g., less than or equal to0.75% or 0.09%).

Mannan content greater than or equal to 4% (e.g., greater than or equalto 4%, 5%, 6%, or 7%). For example, the mannan content can be within arange from 4% to 8% or within a range from 5% to 7%.

R18 greater than or equal to 88% (e.g., greater than or equal to 89%)and/or less than or equal to 92% (e.g., less than or equal to 91% or90%).

Resultant CMC color less than or equal to 5 (e.g., less than or equal to3).

Resultant CMC turbidity less than or equal to 25 ntu (e.g., less than orequal to 5 or 0.5 ntu).

Resultant CMC viscosity greater than or equal to 59 cP (e.g., greaterthan or equal to 60, 90, 120, or 150 cP).

Total transition metal content less than or equal to 20 ppm. The ironcontent can be less than or equal to 5 ppm. The copper content can beless than or equal to 2 ppm. The calcium content can be less than orequal to 150 ppm (e.g., less than or equal to 60 ppm) and/or greaterthan or equal to 30 ppm (e.g., greater than or equal to 50 or ppm).Transition metals are often undesirable in pulp because, for example,they can accelerate the degradation of cellulose in etherificationprocesses.

Water retention value greater than or equal to 1.0 g/g (e.g., greaterthan or equal to 1.1, 1.2, or 1.3 g/g) and/or less than or equal to 1.4g/g.

Xylan content greater than or equal to 4% (e.g., greater than or equalto 5%, 6%, or 7%) and/or less than or equal to 16%. For example, thexylan content can be within a range from 4% to 16%, within a range from5% to 8%, or within a range from 6% to 7%.

EXAMPLES

The following experimental examples are provided to illustrate certainparticular embodiments of the disclosure. It should be understood thatadditional embodiments not limited to the particular features describedare consistent with the following experimental examples.

Referenced Commercial Products

9H4F: Aqualon 9H4F high-viscosity (DS=0.95) CMC from Ashland, Inc.

NB416: Pine-derived, fluff-grade Kraft wood pulp obtained fromWeyerhaeuser Company mill in New Bern, NC.

NB421: Pine-derived, ether-grade Kraft wood pulp obtained fromWeyerhaeuser Company mill in New Bern, NC.

PW416: Pine-derived, fluff-grade Kraft wood pulp obtained fromWeyerhaeuser Company mill in Port Wentworth, GA.

Sulfite1 and Sulfite2: Sulfite-processed, dissolving-grade,spruce-derived pulp from Borregaard ChemCell.

The Weyerhaeuser pulps referenced herein are generally sold in sheet(e.g. roll or bale) or other dried form. The following ExperimentalExamples also make reference to never-dried samples of these pulps, suchas samples obtained from various points in the bleaching process orotherwise prior to drying (such as from the press section).

Experimental Example 1: Epoxide Crosslinked Pulps

The starting material for preparing crosslinked pulp in this example wasPW416 pulp obtained from the extraction stage (EOP) as a 38% solids(after lab centrifugation) wet lap. The pulp was pre-warmed to 75° C. Ina plastic bag, 52.6-gram never-dried (corresponding to 20-gram OD)samples of the pulp were mixed with warm water (75° C.), differentcrosslinkers, and NaOH (pH 11 to 13) at different pulp consistencies asshown in Tables 1 and 2. Tables 1 and 2 list the properties of thecrosslinked pulp samples and the corresponding CMC. For comparison,uncrosslinked PW416 pulp from the EOP stage was found to have aresultant CMC viscosity of 42 cP.

The following polyepoxide crosslinkers were tested: GE-30 (trimethylolpropane triglycidyl ether polymer (TMPTGE)) and GE-31 (trimethylolethane triglycidyl ether polymer) from CVC Thermoset. Glyceroldiglycidyl ether (GDE) from Aldrich. Denacol EX811 and EX810 (bothethyleneglycol diglycidyl ethers (EGDE)), Denacol EX313 (glycerolpolyglycidyl ether (GPE)), Denacol EX314 (glycerol triglycidyl ether(GTE)), and EX612 (sorbitol polyglycidyl ether) from Nagase Chemitex.HELOXY modifier 505 (castor oil polyglycidyl ether (M505)), HELOXYmodifier 48 (trimethylol propane triglycidyl ether, M48 (TMPTGE)),HELOXY modifier 67 (1,4-butanediol diglycidyl ether, M67 (BDDE)), andHELOXY modifier 68 (neopentyl glycol diglycidyl ether, M68) fromMomentive. D.E.R. 736 Epoxy Resin (D736), polypropylene glycol,chloromethyloxirane polymer from Dow Chemical, diethylene glycoldiglycidyl ether (DEGDE), 1,3-dichloro-2-hydroxypropanol (DCP), GPE,BDDE, EGDE, and TMPTGE from other suppliers.

TABLE 1 Different Crosslinkers and Crosslinking Conditions CrosslinkingCondition Crosslinker Pulp Crosslinked Pulp Mn/water Viscosity cons.NaOH Time Cuen FB % used solubility WPE (cP) (%) (%) (hr.) soluble? (cP)1% GE-30 302.4/NG 135-150 100-200 10 2.0 2 yes 167 2% GE-30 302.4/NG135-150 100-200 10 2.0 2 yes 154 2% GE-30 302.4/NG 135-150 100-200 202.3 2 Not 100% 517 2% GE-31 288.3/NG 150-170 200-300 20 2.3 2 Not 100%496 3.5% EX 313 >204/partial 141 150 20 2.5 2 yes 199 3.5% EX314 >204/partial 144 170 20 2.5 2 Not 100% 333 3.5% EX 810 ~174/soluble113 20 20 2.5 2 Not 100% 418 4.5% EX 810 ~174/soluble 113 20 18 2.5 2Not 100% 810 3.5% EX 811 ~174/partial 132 20 20 2.5 2 Not 100% 188 4.5%EX 811 ~174/partial 132 20 18 2.5 2 Not 100% 800 3.5% M48 302.4/NG138-154 120-180 20 2.5 2 Not 100% 366 4.5% M48 302.4/NG 138-154 120-18018 2.5 2 Not 100% 905 4.5% M67 202.3/insol. 123-147 20-30 18 2.5 2 Not100% 916 4.5% M68 216.3/NG 130-145 13-25 18 2.5 2 Not 100% 900 4.5% EGDE~174/partial 129-139 15-25 18 2.5 2 Not 100% 880 5.5% EGDE ~174/partial129-139 15-25 10 2.6 1 yes 375 5.0% GPE >204/insol. 143-154 100-200 182.5 2 Not 100% 503 4.5% TMPTGE 302.4/NG 135-147  90-180 18 2.5 2 Not100% 610 4.5% BDDE 202/partial 122-136 10-20 18 2.5 2 Not 100% 910 4.5%GDE 204.2/NG 102.11 — 18 2.5 2 Not 100% 1700 4.5% DCP 129/soluble — — 182.5 2 Not 100% 351 4.5% D736 246.3/partial 175-205 30-60 18 2.5 2 yes160 4.5% DEGDE 218.3/soluble ~110 — 18 2.5 2 yes 123 5.0% EX612390/partial 166 11900 18 2.5 2 yes 190 2.0% M505 >933/NG 500-650 250-50020 2.3 2 yes 156 Crosslinker condition CMC properties Crosslinker PulpViscosity (cP) Mn/water Viscosity Cons. NaOH, Time of 0.5% CMC % usedsolubility WPE (cP) (%) (%) (hr.) Solution 7.5% GPE >204/insoluble143-154 100-200 11.8 2.0 1 113 7.5% EX314 >204/partial 144 170 11.8 2.01 95 7.5% EGDE ~174/partial 129-139 15-25 11.8 2.0 1 85 7.5% EX810~174/soluble 113 20 11.8 2.0 1 78 7.5% PEGDE 526/soluble 263 — 10 5.0 149

The results shown in Table 1 indicate that polyepoxides D736(dipropyleneglycol diglycidyl ether), DEGDE (diethyleneglycol diglycidylether), EX612 (sorbitol polyglycidyl ether), M505 (castor oilpolyglycidyl ether), PEGDE (poly(ethylene glycol) diglycidyl ether withMn of 525) were not good candidates for crosslinking to produce pulpwith high intrinsic viscosity. Their molecules have five or more thanfive linear chain atoms between two glycidyl ether function groups.

Among the tested crosslinkers having five or more than five linear chainatoms between two glycidyl ether function groups, DEGDE and PEGDE arehighly soluble in water. D736 is partially water soluble. EX612 and M505have negligible solubility (NG) in water, but have high molecular weight(>500), high weight per epoxide (>175) and/or high viscosity (>500 cP).Thus, at least some polyepoxides with the following properties may beuseful for crosslinking pulp in accordance with embodiments of thepresent technology: molecular weight less than or equal to 500, weightper epoxide less than or equal to 175, viscosity less than or equal to500 cP, and molecular structures in which there are fewer than fivelinear chain carbon atoms between two glycidyl ether function groups.

The results also showed that crosslinkers well suited for producingcrosslinked pulp with high resultant CMC viscosity had fewer than fivelinear chain atoms between two glycidyl ether function groups. By way oftheory and not wishing to be bound by theory, these crosslinkers maypenetrate the cellulose structure more readily than crosslinkers havinglonger chains.

It can be advantageous for crosslinkers to be insoluble or onlypartially soluble in water. Insoluble or only partially solublecrosslinkers, for example, may contact and react with cellulose fibersmore readily than crosslinkers that are highly soluble in water.Polyepoxides with low or no water solubility produced better resultsthan polyepoxides with greater water solubility. For example, GPE hasalmost the same structure as EX314 except the latter is modified to havehigher water solubility; and EGDE has the same structure as EX810,except the latter is modified to have higher water solubility. Table 2shows additional results using GPE (water insoluble: Mn>204, <500) withthe EOP pulp (NB416 or PW416) at different consistencies. Intrinsicviscosity (IV*) was calculated from a model based on commercial sampleshaving known intrinsic viscosities. Supporting data for the model can befound in the lower portion of Table 2. The resulting model was IV*=717.2ln(A)−1817.3 (R²=0.9988). The results shown in Table 2 indicate thathigher crosslinking consistency will produce better results.

TABLE 2 Different Consistencies Pulp FB Pulp Pulp Pulp 0.5% CMC Startingcons. visc. IV* R18 Xylan Mannan viscosity Epoxide NaOH Pulp (%) (mPa ·s) (mL/g) (wt. %) (wt. %) (wt. %) (cP) (%) (%) NB416 — 263 922 86.7 8.56.2 45.6 0.0 — (control) NB416 11.5 228 897 86.6 8.3 6.2 44.0 7.6 0NB416 10.0 2340 1514 89.4 8.6 6.3 104.0 7.5 5 NB416* 12.0 525 1622 90.48.5 6.3 121.0 7.5 5 NB416 13.9 250 1823 91.2 8.4 6.2 160.0 7.5 5 NB416**20.0 insol. 2160 92.8 8.4 6.3 256.0 7.5 5 NB416 10.6 364 1311 88.4 8.56.2 78.4 3.8 5 NB416 14.0 788 1402 89.2 8.6 6.3 89.0 3.8 5 NB416 20.01480 1418 90.5 8.4 6.2 91.0 3.7 5 PW416 — 100 843 87.4 8.8 6.5 40.8 — —(control) PW416 11.2 396 1110 90.1 8.8 6.6 59.2 7.7 5 PW416 13.6 10001369 90.9 8.7 6.4 85.0 7.7 5 Below: commercial samples used to calculatepulp IV from 0.5% CMC solution viscosity (A) Pulp Pulp Pulp 0.5% CMCCommercial Known FB IV* R18 Xylan Mannan viscosity Sample IV (mPa · s)(mL/g) (wt. %) (wt. %) (wt. %) (cP) — NB421 910 — 922 87 8.2 6.2 45.6 —Sulfite1 1230 1180 1217 92.3 3.7 2.9 68.8 — Sulfite2 1435 2440 1426 94.93.3 1.7 92 — Cotton linter 1795 11600 1805 99.5 <1 <1 156 — *Thecorresponding crosslinked pulp is referred to elsewhere in thisdisclosure as “Kraft3” **The corresponding crosslinked pulp is referredto elsewhere in this disclosure as “Kraft2”

Experimental Example 2: Bleached EGDE Crosslinked Pulp (O-D-E-X)

In this example, partially water soluble ethyleneglycol diglycidyl ether(EGDE) was used as the crosslinker. The starting material for preparingcrosslinked pulp in this example was PW416 pulp obtained from theextraction stage as a 38.5% solids (after lab centrifugation) wet lap.60-gram (OD) samples of the pulp were mixed with water, EGDE, and NaOHso the final concentrations of EGDE and NaOH were 8.8% and 4.8%,respectively, and the final pulp consistency was 10%. The pulp mixturewas hand mixed for a few minutes, filtered to remove half the liquid andthen reacted for 2 hr. at 75° C. Half of the resulting crosslinked pulpwas thoroughly water washed and was then made into a sheet with basisweight of 747 g/m² and a density of 0.53 g/cm³. The sheet had abrightness of 79.3% and a falling ball viscosity of about 1,350 cP. Thecrosslinked pulp was not 100% soluble in cupriethylenediamine. CMC fromthe pulp had a 0.5% solution viscosity of 86 cP (Sample 1A in Table 5).The other half of the crosslinked pulp (not washed) was bleached withH₂O₂ (0.76% by dry pulp weight) at 76° C. for 30 minutes (Sample 1B inTable 5). This pulp was washed and made into a sheet with a basis weightof 746 g/m² and a density of 0.54 g/cm³. The pulp sheet had a brightnessof 82.3% and a FB viscosity of 1,270 cP. CMC from the pulp had a 0.5%solution viscosity of 84 cP. Other properties are summarized in Table 5.

Experimental Example 3: Bleached GTE Crosslinked Pulp (O-D-E-X-D)

In this example, glycerol triglycidyl ether (GTE) was used as thecrosslinker. The starting material for preparing crosslinked pulp inthis example was NB416 pulp obtained from the extraction stage (NB416EOP) as a 38.5% solids (after lab centrifugation) wet lap. 60-gram (ovendried) samples of the pulp were mixed with water, GTE and NaOH forcrosslinking at 75° C. for one hour. The crosslinked pulp was then mixedwith bleaching chemicals (ClO₂ or H₂O₂) for reaction at 75° C. for 45minutes. Bleached samples had increased brightness (78% to 86%) and CMCviscosity (Table 3).

TABLE 3 Bleaching Pulp 0.5% CMC Starting Consistency HemicelluloseBleaching Brightness viscosity Pulp % (wt. %) Agent (%) (cP) Epoxide %NaOH % NB416 EOP — 14.7 — — 45.6 — — (non-crosslinked control) NB416 EOP11.8 14.5 — 78.6 120.0 7.5 5.0 (control) NB416 11.8 14.8 0.5% ClO₂ 86.394.9 7.5 5.0 NB416 11.8 14.9 0.25% ClO₂ 85.2 109.8 7.5 5.0 NB416 11.814.8 0.5% H₂O₂ 82.0 115.7 7.5 5.0 NB416 11.8 14.5 1% H₂O₂ 83.0 116.9 7.55.0

Experimental Example 4: Bleached EGDE Crosslinked Pulp (O-D-E-D-X)

In this example, ethyleneglycol diglycidyl ether (EGDE) was again usedas the crosslinker. The starting material for preparing crosslinked pulpin this example was NB416 pulp obtained from the extraction stage as a38.5% solids (after lab centrifugation) wet lap. 60-gram (oven dried)samples of this pulp were mixed with water, EGDE, and NaOH so that theconcentrations of EGDE and NaOH were 11% and 5.4%, respectively, and thepulp had a consistency of 10%. The pulp mixture was hand mixed for a fewminutes. Half of the liquid was filtered so the final concentrations ofEGDE and NaOH in the pulp were 4.9% and 2.7%, respectively, and the pulpconsistency was 20%. The mixture was then reacted for 2 hr. at 80° C.The resulting crosslinked pulp was thoroughly water washed, and a TAPPIhandsheet was made with a basis weight of 65 g/m² and a density of 0.65g/cm³. The sheet had a brightness of 87.8% and a falling ball viscosityof 2,710 cP. The pulp was not totally soluble in cupriethylenediamine.CMC from the pulp had a 0.5% solution viscosity of 126 cP (Sample 2A inTable 5). The other half of the wet crosslinked pulp was treated at 10%consistency with H₂O₂ (1% by dry pulp weight) and NaOH (0.5% by dry pulpweight) at 75° C. for 1 hour. The bleached pulp was washed and a TAPPIhandsheet was made with a basis weight of 65 g/m² and a density of 0.68g/cm³. The crosslinked pulp had a falling ball viscosity of 283 cP and abrightness of 89.4%. CMC from the pulp had a 0.5% solution viscosity of107 cP (Sample 2B in Table 5).

Experimental Example 5: Crosslinking Efficacy at Different Temperatures

The procedure from Experimental Example 4 was repeated to make moresamples using EGDE as the crosslinker and with the final concentrationsof EGDE and NaOH in pulp of about 4.9% and 2.7%, respectively. The pulpwas crosslinked at a consistency of 20%. The crosslinked pulp waswashed, but not bleached. The falling ball viscosity and CMC propertiesof the crosslinked pulp are summarized in Table 4 below (Samples A1-A7).All of the samples were not fully soluble in cupriethylenediamine. Largeamounts of crosslinked pulp were not dissolved in cupriethylenediamine.

TABLE 4 Crosslinking Temperatures Pulp FB 0.5% CMC Temp. visc. viscosityIV* Sample (° C.) Crosslinker (mPa · s) (cP) (mL/g) A1 70 EGDE 2200 1291668 A2 75 EGDE 2680 177 1895 A3 75 EGDE 1840 161 1827 A4 80 EGDE 301095 1449 A5 80 EGDE 3450 88 1394 A6 80 EGDE 4190 90 1410 A7 85 EGDE  76070 1230 Below: commercial non-crosslinked DWP and kraft pulp forcomparison Sulfite 1 — — 1180 68.8 1217 Sulfite 2 — — 2440 92 1426 PW416— —  160 40  828 NB421 — —  242 46  929

For the crosslinked pulp listed in Table 4, pulp FB viscosity was not agood indicator of the corresponding CMC viscosity, especially at veryhigh crosslinking densities. At very high crosslinking densities,crosslinked pulp actually showed low FB viscosity because thecrosslinked pulp did not dissolve completely in cupriethylenediamine.The CMC solution viscosities from these very crosslinked pulps, however,was very high and the CMC solutions were clear. The intrinsicviscosities (IV*) for these pulps are listed in Table 4. The temperatureimpact for crosslinking efficacy was clear for the crosslinker tested.Other crosslinkers may have other optimal temperature ranges, such as50° C. to 85° C.

For comparison, two commercial high viscosity dissolving ether gradepulps (Sulfite1 and Sulfite2) and two commercial Kraft pulps (PW416 andNB421) were made into CMC without first crosslinking the pulps. Thesepulps were soluble in cupriethylenediamine and their pulp FB viscositieswere good indications of their CMC viscosities. CMC solutions from thesecommercial sulfite and Kraft pulps were clear, but the solutionviscosities, especially for the CMC solution from the Kraft pulp, wererelatively low. Samples 1A and 1B in Table 5 are described inExperimental Example 2 above. Samples 2A and 2B in Table 5 are describedin Experimental Example 4 above. In addition to the data shown in Table5, CMC from Sample 1B was found to have a lignin content of 0.2 wt. %.For comparison with the CMC data shown in Table 5, 9H4F was found tohave a CMC color of 0.12, a 0.013% CMC turbidity of 0.12 ntu, and a0.67% CMC turbidity of 1.1 ntu.

TABLE 5 Pulp and CMC Properties 0.013% 0.67% Pulp FB Pulp Pulp Pulp PulpCMC CMC visc. R18 Pulp Xylan Mannan Lignin CMC Turb. Turb. Sample (cP)(wt. %) brightness (wt. %) (wt. %) (wt. %) Color (ntu) (ntu) 1A (see1350 88.5 79.3 8.6 6.2 0.7 0.0 0.13 0.8 Example 2) 1B (see 1270 88.282.3 8.4 6.1 0.6 0.0 0.11 1.0 Example 2) 2A (see 2710 90.6 87.8 7.8 5.70.5 — — 1.0 Example 4) 2B (see 283 90.3 89.4 7.7 5.7 0.3 — — 1.3 Example4) Below: commercial non-crosslinked DWP and Kraft pulp for comparisonPW416 160 87.3 86.8 8.8 6.5 0.1 0.0 0.11 1.0 NB421 242 87.5 86.5 8.5 6.50.6 0.0 0.10 0.8 Sulfite1 1160 92.3 88.8 3.7 2.9 0.8 0.0 0.10 1.1Sulfite2 2440 94.9 90.3 3.3 1.7 0.4 0.0 0.11 1.1

Experimental Example 6: X-Ray Diffraction of Pulp

X-ray diffraction scans were performed on high viscosity dissolving woodpulp, cotton linter pulp, commercial Kraft pulp (NB421), pulp from theextraction stage of a Kraft process, and corresponding crosslinked pulpsin accordance with embodiments of the present technology. Thecrosslinked pulps were found to have the same cellulose-I crystallinestructure as the starting pulps with peaks at greater than 15° andbetween 21.5° and 22.5°. The main peaks for the crosslinked pulp shiftedslightly to higher diffraction angles (bleached pulp from 21.5° to22.4°, extraction-stage pulp from 21.7° to 22.2°). Cellulose-IIcrystalline structure has a peak at 12.5° and between 20° and 21.5°. Forcellulose-I, peaks at diffraction angles of 22° and 18° are crystallineand amorphous peaks, respectively. For cellulose-II, peaks atdiffraction angles of 19° and 15° are crystalline and amorphous peaks,respectively. Table 6 summarizes the x-ray diffraction data.

TABLE 6 X-Ray Diffraction Pulping IV Crystalline Crystallinity R18Sample Species Type (mL/g) Structure Index (%) (%) 1A (see S Pine KraftNot 100% I 66 88.2 Example 2) soluble 1B (see S Pine Kraft Not 100% I 7288.2 Example 2) soluble Below: commercial non-crosslinked DWP and Kraftpulp for comparison DWP spruce sulfite 1230 I 80 92.3 DWP spruce sulfite1435 I 81 94.9 DWP Cotton linter Soda 1795 I 88 99.5 DWP Cotton linterSoda 200 I 89 96.3 NB421 S Pine Kraft 890 I 75 87.5

Experimental Example 7: Freeness and Other Properties of CrosslinkedPulp

Kraft1 was prepared using the same procedure as Sample 1A inExperimental Example 2 except that the EGDE concentration was 4.6% andthe NaOH concentration was 2.2%. The crosslinked pulp was tested forfreeness and brightness. The results are listed in the Table 7. Thecrosslinked pulp had similar brightness and freeness, thus similardrainage, as the starting pulp.

TABLE 7 Pulp and CMC Properties CMC (DS: 1.1) properties Pulp FB 0.5%CMC Free Swell Centrifugal Brightness Freeness viscosity viscosityCapacity Capacity Pulp (%) (mL) (cP) (cP) (g/g) (g/g) Kraft1 86.6 7702640 140 14.9 13.7 Below: commercial non-crosslinked DWP and Kraft pulpfor comparison Sulfite2 90.3 ~700 2440 92 11.8 13.3 NB421 86.5 746 24246 11.7 9.5

CMC from the crosslinked Kraft pulp also had higher free swell andcentrifugal capacities than commercial uncrosslinked ether grade pulpfrom a sulfite process (Sulfite2) or a Kraft process (NB421).

Experimental Example 8: WRV, Wet Bulk, and Other Properties ofCrosslinked Pulp

More samples of crosslinked pulp were tested for wet bulk and otherproperties. The results of this testing are shown in Table 8. The testedsamples were based on samples described in European Patent ApplicationPublication No. 0399564 (Sample 1) and U.S. Pat. No. 8,722,797 (Sample2), which are incorporated herein by reference in their entireties.Sample 1 is 1,3-dichloro-2-hydroxypropanol (DCP) crosslinked Kraft pulp.Sample 2 is polycarboxylic acid crosslinked Kraft pulp.

KraftA was prepared in the same procedure as Sample 1A in ExperimentalExample 2, except that the pulp consistency was 16%, the EGDEconcentration was 4.6% relative to pulp dry weight, and the NaOHconcentration was 2.2%. The wet bulk and capacity of the Kraft1 pulpwere lower than those of Samples 1 and 2, and similar to those ofregular fluff pulp (NB416). The Kraft1 pulp, however, produced muchhigher CMC solution viscosity than both regular fluff pulp and Sample 1.Sample 2 had lower R18 than the uncrosslinked control pulp due tobreaking of the relatively weak ester crosslinks. The Kraft1 sample andSample1 had higher R18 than the uncrosslinked control pulp because theether crosslinks were relatively stable in the R18 testing process.Sample 1 was overly crosslinked and therefore unsuitable for producingcellulose ether.

TABLE 8 Pulp and Resultant CMC Properties 0.5% 0.5% Pulp Pulp CarboxylWet Bulk CMC CMC Ash WRV FB Visc. Content (0.6 kPa R18 viscosity Turb.Sample (%) (g/g) (cP) (meq/100 g) Bond (cc/g)) (wt. %) (cP) (cP) NB4160.09 1.08 196 3.2 — 11.97 87.4 45 0.8 (control) KraftA — 1.35 — — ether— — 435 3.6 Kraft1 0.16 1.30 2640 4.7 ether 11.43 89.9 161 1.0 (seeExample 7) Kraft2 — 1.23 — — ether — 92.8 256 2.0 (see footnote in Table2) Kraft3 — 1.09 525 — ether — 90.4 121 1.0 (see footnote in Table 2)Sample 1 0.16 0.70 insoluble 3.7 ether 14.29 99.0 30 — Sample 2 0.950.36 insoluble 19.0 ester 14.19 86.6 unstable 31

Experimental Example 9: HPLC Spectrum for Hydrolyzed Crosslinked Pulp

KraftA from Experimental Example 8 was hydrolyzed for HPLC testing. Anew sugar peak was observed indicating crosslinking between cellulosefibers in the sample.

Experimental Example 10: Metals in Crosslinked Pulp and DCM ExtractionResidue

Additional samples were prepared using the procedure of ExperimentalExample 4, except that the final crosslinker and NaOH concentrationswere 4.7% and 2.6%, respectively. The consistency was 19% and thetemperature was 75° C. Reaction times are listed in Table 9. Afterwashing the pulp, the metal content, falling ball viscosity, andresultant CMC viscosity were tested.

TABLE 9 Pulp and Resultant CMC Properties Crosslinker Pulp FB 0.5% CMCTime Ca Cu Fe Na Mg viscosity viscosity (hr.) (ppm) (ppm) (ppm) (ppm)(ppm) (cP) (cP) 1 120 2.5 1 490 20 3180 210 1.5 90 0.6 <1 470 20 4190161 2 60 0.5 1 270 10 3450 177

Low calcium content and low transition metal content may be importantfor certain end uses. DCM residue from the crosslinked pulp was alsotested. The crosslinked pulp was found to have extractive of less than0.01%. Normal bleached pulp without crosslinking also had DCM extractiveof less than 0.01%. IR spectra for the residue did not show anycrosslinker.

Experimental Example 11: Fiber Morphology

Scanning electron microscopy analysis of crosslinked pulp showedearlywood pine. The fiber analysis showed that crosslinking changed thefiber morphology. Coarseness, curl, and kink index of the crosslinkedpulp increased with crosslinking consistency (Table 10). Highercoarseness, curl, and kink may be desirable, such as for increasingfiber accessibility during derivatization reactions.

TABLE 10 Fiber Analysis 0.5% CMC Kink Kink Solid viscosity CoarsenessCurl Angle Index (wt %) Crosslinker (cP) (mg/100 m) Index (Deg/mm)(1/mm) Control — 46 23.5 0.202 72 2 11.5 EGDE* 44 22.9 0.236 84 2.4 10EGDE 104 24.7 0.261 96 2.6 13.9 EGDE 160 27.1 0.292 105 2.8 20 EGDE 25628.9 0.341 115 3.2 10.6 EGDE 78.4 23.1 0.221 77 2.2 14 EGDE 89 24.50.264 92 2.6 20 EGDE 97 23.9 0.299 104 2.9 *No catalyst (NaOH), seeTable 3 for other conditions.

Experimental Example 12: Process to Prepare Crosslinked Cellulose

PW416 pulp was obtained from the extraction stage as a 38% solids (afterlab centrifugation) wet lap. 20-gram (OD) samples of this pulp werepre-warmed to 80° C. and mixed in a plastic bag with warm water (80°C.), crosslinkers, and NaOH so that the final concentrations ofcrosslinkers and NaOH were 2.0% and 2.3%, respectively. The pulpconsistency was 10, 15 and 20% to produce samples L, M and N. All of thecrosslinking mixtures had a pH greater than 11. Crosslinking was allowedto occur for 2 hours.

NB421 pulp from couch trim (fully-bleached, never-dried) was obtained asa 32.8% solids wet lap. This pulp was used to prepare samples O, P, andQ. The same procedure described above for samples L, M and N was usedand samples O, P, and Q had corresponding consistencies of 10, 15, and20%, respectively. Sample R had a crosslinking temperature of 60° C. Theproperties of these samples are shown in Table 11 along with those of acontrol pulp from a mill production. Crosslinking was allowed to occurfor 2 hours. 0.67% CMC and 1.33% CMC viscosities are shown in Table 11,whereas elsewhere in this disclosure, 0.5% CMC viscosities are provided.

TABLE 11 DCP Crosslinked Pulp 0.67% CMC 1.33% CMC Solid DCP NaOH FB Cuenviscosity viscosity Sample (wt. %) (wt. %) (wt. %) (cP) solubility (cP)(cP) L 10 2.0 2.3 244 almost 64 380 M 15 2.0 2.3 387 almost 104 570 N 202.0 2.3 1410 low 160 900 O 10 2.0 2.3 324 100% 76 392 P 15 2.0 2.3 465almost 120 664 Q 20 2.0 2.3 1240 low 140 900 R 20 2.5 2.6 1176 low — —Below: commercial non-crosslinked Kraft pulp for comparison NB421 — 0% —242 100% 88 528

Under certain conditions (e.g., 15 to 20% pulp consistency), crosslinkedKraft pulp may have much higher (e.g., greater than 100% higher) fallingball viscosity than the starting pulp. These crosslinked Kraft pulpswere not 100% soluble in Cuen. But, surprisingly, these crosslinkedpulps were found to generate water soluble CMC solutions that were clearand had higher viscosities than CMC solutions produced from NB421 Kraftpulp with falling ball viscosity of 242 cP. The crosslinked pulps didnot undergo extraction and, therefore, had high hemicellulose contentand cellulose-I crystal structure like standard Kraft pulp. Crosslinkedbleached Kraft pulp had high brightness. Crosslinked pulp from theextraction stage also had high brightness. It can be advantageous forcrosslinking to be performed at high pulp consistency (e.g., 11 to 30%),with pH from 9 to 14, and a temperature from 50 to 85° C.

Experimental Example 13: Pulp Intrinsic Viscosity and Resultant CMCShear Rate

Three control pulps (Sulfite1, Sulfite2, and Cotton Linters), onecommercial pulp (Aqualon 9H4F from Ashland Company), and two crosslinkedKraft pulps (KraftA and KraftB which was prepared as Kraft in table 2except the consistency was 17%) were used to form 1% and 0.5% CMCsolutions. The intrinsic viscosities of the CMC solutions from thesepulps under different shear rates are shown in Table 12.

TABLE 12 Pulp Intrinsic Viscosity and Resultant CMC Shear Rate Spindle(5) RPM Spindle (2) RPM SCAN (1% CMC solution) (0.5% CMC solution)Sample IV 5 10 20 50 100 5 10 20 50 100 KraftB — 1140 1216 1140 927 741190 219 217 196 176 KraftA — 3572 2964 2413 1710 1302 676 608 527 413337 Below: commercial non-crosslinked DWP for comparison Sulfite1 1230 00 0 137 182 0 27 38 69 87 Sulfite2 1435 0 0 152 247 281 0 59 74 92 109Cotton 1795 0 532 722 732 670 86 124 144 154 156 Linters Below:commercial CMC for comparison 9H4F — — 480 660 670 600 130 160 174 175170

Experimental Example 14: Washing Procedures and AOX Content

The AOX content of a pulp indicates the amount of adsorbable organichalides (or halogenated compounds) it contains. As a reference, the AOXcontent of two different samples of NB421 (with SCAN IV of 1460 ml/g)was found to be 2.8 and 5.3 ppm, respectively. The AOX content of acommercial cotton linter pulp with SCAN IV of 1760 ml/g was found to be4.6 ppm. Although higher AOX levels were not found to be associated withany detrimental effects on desired properties of the crosslinked pulpsdescribed herein or resultant CMC produced therefrom, AOX content higherthan that usually found in high-purity pulps may represent a downstreamcost in treatment and handling of water used in processing the pulp.

A sample of never-dried NB416 Kraft wood pulp (Weyerhaeuser Company)containing never-dried fibers equivalent to 80 g OD fibers, with fallingball viscosity of 200 cP, hemicellulose content of 15%, ISO Brightnessof 86% and R18 content of 87.5%, was warmed to 75° C. in a plastic bag.To the pulp was added a mixture of warm (75° C.) water, crosslinker(GTE), and NaOH sufficient to produce a pulp consistency of 19%, withNaOH and GTE dosage at 4.9% and 7.4% (based on OD weight of the pulp)respectively. The mixture was placed in an oven at 75° C. for one hourand allowed to react.

After the crosslinking reaction, the mixture was dispersed in a Waringblender, then neutralized with acetic acid, washed, and then filteredwith cold (about 20° C.) water twice to obtain wet pulp, which was thencentrifuged before being allowed to air dry.

The AOX content of the crosslinked pulp was determined to be 32 ppm. TheR18 value of the pulp was 92%. CMC from the crosslinked pulp wasprepared. The CMC solution was found to have an intrinsic viscosity of230 cP.

The same procedure above was repeated except that instead of beingdisintegrated in a Waring blender, the crosslinked pulp was hand-stirredprior to neutralization and washing. The crosslinked pulp produced withthis different washing operation exhibited an AOX content of 1050 ppm.

The same procedure above was repeated a second time except that thefinal pulp consistency was 12%, and the washing operation includedhand-stirring instead of high-energy dispersion in a Waring blender. Thecrosslinked pulp produced in this manner exhibited an AOX content of 724ppm.

Experimental Example 15: Use of Dried Pulp

An 80 g OD sample of dried NB416 Kraft wood pulp (Weyerhaeuser Company)with falling ball viscosity of 200 cP, hemicellulose content of 15%, ISOBrightness of 86% and R18 content of 87.5%, was warmed to 75° C. in aplastic bag. To the pulp was added a mixture of warm (75° C.) water,crosslinker (M67), and NaOH sufficient to produce a pulp consistency of12.9%, with NaOH and M67 dosage at 5.0% and 7.5% (based on OD weight ofthe pulp) respectively. The mixture was placed in an oven at 75° C. forone hour and allowed to react.

After the crosslinking reaction, the mixture was neutralized with aceticacid, washed and then filtered with warm (about 50° C.) water twice toobtain wet pulp, which was then centrifuged before being allowed to airdry.

The AOX content of the crosslinked pulp was determined to be 7.8 ppm.The WRV of the pulp was 1.17 g/g and the R18 value of the pulp was91.3%. The lignin content was found to be less than 1%, and the pulp hadzero cellulose-II structure. CMC from the crosslinked pulp was prepared.The 0.5% CMC solution was found to have an intrinsic viscosity of 89 cPand an equivalent SCAN IV of 1438 ml/g.

The same procedure was repeated, except that the crosslinked pulp wasdried at a temperature of 100° C. The WRV of the pulp was 1.01 g/g, andthe resultant CMC viscosity was 82 cP.

CONCLUSION

This disclosure is not intended to be exhaustive or to limit the presenttechnology to the precise forms disclosed herein. Although specificembodiments are disclosed herein for illustrative purposes, variousequivalent modifications are possible without deviating from the presenttechnology, as those of ordinary skill in the relevant art willrecognize. In some cases, well-known structures and functions have notbeen shown and/or described in detail to avoid unnecessarily obscuringthe description of the embodiments of the present technology. Althoughsteps of methods may be presented herein in a particular order, inalternative embodiments the steps may have another suitable order.Similarly, certain aspects of the present technology disclosed in thecontext of particular embodiments can be combined or eliminated in otherembodiments. Furthermore, while advantages associated with certainembodiments may have been disclosed in the context of those embodiments,other embodiments may also exhibit such advantages, and not allembodiments need necessarily exhibit such advantages or other advantagesdisclosed herein to fall within the scope of the present technology.

Throughout this disclosure, the singular terms “a,” “an,” and “the”include plural referents unless the context clearly indicates otherwise.Similarly, unless the word “or” is expressly limited to mean only asingle item exclusive from the other items in reference to a list of twoor more items, then the use of “or” in such a list is to be interpretedas including (a) any single item in the list, (b) all of the items inthe list, or (c) any combination of the items in the list. Additionally,the terms “comprising” and the like are used throughout this disclosureto mean including at least the recited feature(s) such that any greaternumber of the same feature(s) and/or one or more additional types offeatures are not precluded. It should be understood that such terms donot denote absolute orientation. Reference herein to “one embodiment,”“an embodiment,” or similar formulations means that a particularfeature, structure, operation, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of thepresent technology. Thus, the appearances of such phrases orformulations herein are not necessarily all referring to the sameembodiment. Furthermore, various particular features, structures,operations, or characteristics may be combined in any suitable manner inone or more embodiments of the present technology.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Pulp, comprising:crosslinked cellulose fibers, wherein the pulp has: a resultantcarboxymethyl cellulose (CMC) viscosity greater than or equal to 60centipoise (cP), a brightness greater than or equal to 75%, and a waterretention value (WRV) greater than or equal to 1.0 g/g.
 2. The pulp ofclaim 1, wherein the pulp has: a resultant CMC viscosity greater than orequal to 60 cP, a brightness greater than or equal to 75%, and a WRVgreater than or equal to 1.1 g/g.
 3. The pulp of claim 1, wherein thepulp is a kraft pulp.
 4. The pulp of claim 1, wherein the pulp isbleached.
 5. The pulp of claim 1, wherein the pulp is at least partiallyinsoluble in cupriethylenediamine (cuen).
 6. The pulp of claim 1,further comprising less than or equal to 0.09% lignin by oven-driedweight of the crosslinked cellulose fibers.
 7. The pulp of claim 1,further comprising greater than or equal to 10% hemicellulose byoven-dried weight of the crosslinked cellulose fibers.
 8. The pulp ofclaim 1, having a resultant CMC viscosity greater than or equal to 90cP.
 9. The pulp of claim 1, having a resultant CMC color less than orequal to
 5. 10. The pulp of claim 1, having a resultant CMC turbidityless than or equal to 25 Nephelometric Turbidity Units (NTU).
 11. Thepulp of claim 1, having a WRV less than or equal to 1.4 g/g.
 12. Thepulp of claim 1, having an R18 value greater than or equal to
 88. 13.The pulp of claim 12 having an R18 value less than or equal to
 92. 14.The pulp of claim 1, having a falling ball viscosity greater than orequal to 200 cP.
 15. The pulp of claim 1 having no cellulose-II asdetermined by x-ray crystallography.
 16. The pulp of claim 1, having acrystallinity index less than or equal to 80%.
 17. The pulp of claim 16having a crystallinity index less than or equal to 75%.
 18. The pulp ofclaim 1 having a brightness greater than or equal to 80%.
 19. The pulpof claim 18 having a brightness less than or equal to 88.5%.
 20. Thepulp of claim 1, having a basis weight of greater than or equal to 500g/m².
 21. The pulp of claim 20 having a basis weight less than or equalto 1200 g/m².
 22. The pulp of claim 1 having a density greater than orequal to 0.20 g/cm³.
 23. The pulp of claim 1 having a freeness greaterthan or equal to 700 mL.
 24. The pulp of claim 1 having an AOX contentless than or equal to 200 ppm.
 25. The pulp of claim 3 having an AOXcontent less than or equal to 100 ppm.
 26. The pulp of claim 3 having anAOX content less than or equal to 50 ppm.
 27. The pulp of claim 3 havingan AOX content less than or equal to 20 ppm.
 28. The pulp of claim 1,wherein the crosslinked cellulose fibers include crosslinks derived froma glycidyl ether crosslinker having two or more glycidyl groups.
 29. Thepulp of claim 1, wherein the crosslinked cellulose fibers includecrosslinks derived from a glycidyl ether crosslinker having a weightaverage molecular weight within a range from 174 to
 500. 30. The pulp ofclaim 1, wherein the crosslinked cellulose fibers include crosslinksderived from a glycidyl ether crosslinker having a weight per epoxidewithin a range from 140 to
 175. 31. The pulp of claim 1, wherein thecrosslinked cellulose fibers include crosslinks derived from a glycidylether crosslinker having a first glycidyl group, a second glycidylgroup, and either three or four linear chain carbon atoms between thefirst and second glycidyl groups.
 32. The pulp of claim 1, wherein thecrosslinked cellulose fibers are derived from wood.
 33. A pulpcomprising crosslinked cellulose fibers, the pulp having an R18 value ina range of 90% to 92.8%; a kink index in a range of 2.6 mm⁻¹ to 3.2mm⁻¹; and a curl index in a range of 0.261 to 0.341.
 34. The pulp ofclaim 33, wherein the pulp has a WRV in a range of 1.0 g/g to 1.5 g/g.35. The pulp of claim 33, wherein the pulp has a resultant CMC viscositygreater than or equal to 60 cP; a brightness greater than or equal to75%; and a WRV greater than or equal to 1.1 g/g.
 36. The pulp of claim33, wherein the pulp is at least partially insoluble in cuen.
 37. Thepulp of claim 33, further comprising less than or equal to 0.09% ligninby oven-dried weight of the crosslinked cellulose fibers.
 38. The pulpof claim 33, wherein the pulp has a resultant CMC viscosity in a rangeof cP to about 500 cP.
 39. The pulp of claim 33, wherein the pulp has aresultant CMC color less than or equal to
 5. 40. The pulp of claim 33,wherein the pulp has a resultant CMC turbidity less than or equal to 25NTU.
 41. The pulp of claim 33, wherein the crosslinked cellulose fibersinclude crosslinks derived from a glycidyl ether crosslinker having twoor more glycidyl groups.
 42. The pulp of claim 33, wherein the pulp hasa hemicellulose content in a range of 6 wt. % to 20 wt. %
 43. A pulpcomprising crosslinked cellulose, the pulp having a hemicellulosecontent in a range of 6 wt. % to 20 wt. %; a WRV in a range of 1.0 g/gto 1.5 g/g; and a kink index in a range of 2.6 mm⁻¹ to 3.2 mm⁻¹.
 44. Thepulp of claim 43, wherein the pulp has an R18 value in a range of 90% to92.8%.
 45. The pulp of claim 43, wherein the pulp has a curl index in arange of 0.261 to 0.341.
 46. The pulp of claim 43, wherein the pulp hasa resultant CMC viscosity greater than or equal to 60 cP; a brightnessgreater than or equal to 75%; and a WRV greater than or equal to 1.1g/g.
 47. The pulp of claim 43, wherein the pulp is at least partiallyinsoluble in cuen.
 48. The pulp of claim 43, wherein the pulp includesless than or equal to 0.09% lignin by oven-dried weight of thecrosslinked cellulose fibers.
 49. The pulp of claim 43, wherein the pulphas a resultant CMC viscosity in a range of 60 cP to about 500 cP. 50.The pulp of claim 43, wherein the pulp has a resultant CMC color lessthan or equal to
 5. 51. The pulp of claim 43, wherein the pulp has aresultant CMC turbidity less than or equal to 25 NTU.
 52. The pulp ofclaim 43, wherein the crosslinked cellulose fibers include crosslinksderived from a glycidyl ether crosslinker having two or more glycidylgroups.
 53. A pulp blend comprising: a first pulp comprising crosslinkedcellulose fibers, wherein the first pulp has a resultant CMC viscositygreater than or equal to 60 cP, a brightness greater than or equal to75%, and a WRV greater than or equal to 1.1 g/g; and a second pulp. 54.The pulp blend of claim 53, wherein the first pulp is partially solublein cuen.
 55. The pulp blend of claim 53, wherein the first pulp isinsoluble in cuen.
 56. The pulp blend of claim 53, wherein the secondpulp is a dissolving-grade pulp.
 57. The pulp blend of claim 56, whereinthe dissolving-grade pulp has a cellulose content of greater than 90% byoven dried weight.
 58. The pulp blend of claim 53, wherein the firstpulp makes up greater than 20% by cellulose oven-dried weight of thepulp blend.
 59. The pulp blend of claim 53, wherein the first pulp has ahemicellulose content in a range of 6 wt. % to 20 wt %; a WRV in a rangeof 1.0 g/g to 1.5 g/g; and a kink index in a range of 2.6 mm⁻¹ to 3.2mm⁻¹.
 60. The pulp blend of claim 53, wherein the first pulp has an R18value in a range of 90% to 92.8%; a kink index in a range of 2.6 mm⁻¹ to3.2 mm⁻¹; and a curl index in a range of to 0.341.
 61. A crosslinked CMChaving a 0.5 wt. % solution viscosity in a range of 60 cP to 500 cP. 62.The crosslinked CMC of claim 61, wherein the crosslinked CMC has a colorless than or equal to
 5. 63. The crosslinked CMC of claim 61, whereinthe crosslinked CMC has a turbidity less than or equal to 25 NTU. 64.The crosslinked CMC of claim 61, wherein the crosslinked CMC is madefrom a pulp according to claim
 1. 65. The crosslinked CMC of claim 61,wherein the crosslinked CMC is made from a pulp according to claim 10.66. The crosslinked CMC of claim 61, wherein the crosslinked CMCincludes crosslinks derived from a glycidyl ether crosslinker having twoor more glycidyl groups.
 67. A CMC fiber having a 0.5 wt. % solutionviscosity in a range of 60 cP to 500 cP.
 68. The CMC fiber of claim 67,wherein the CMC fiber has a degree of substitution in a range of 1 to 3.69. The CMC fiber of claim 67, wherein the CMC fiber is a crosslinkedCMC fiber including crosslinks derived from a glycidyl ether crosslinkerhaving two or more glycidyl groups.